Ability to provide custom mapper for properties with `"writeOnly": True` or other 'confidential'/'private' property tags

[aaronsteers]

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This feature would enable users to specify a standard rule for fields that are of a specific classification level. For instance, the user could add a rule to hash, nullify, or mask all fields with writeOnly flag, or with data classification of 'pii'.

Related to, and pairs with:

• https://github.com/meltano/sdk/issues/1538